What are good swaddling guidelines?

When swaddled, newborns have just their heads poking out. Place their arms crossed over their chest and immobilised, as they don’t yet know these scary appendages belong to them, let alone how to have any control over them.  At around 6–7 weeks of age (to due date), you could swaddle them tightly enough to keep their flailing arms and legs contained, but they may like their hands up on their chest near their chin (hands exposed near their face). By about 3–4 months old, you can swaddle them with the option of one arm left out, so they can find their thumb or fingers, if you want.